phonet: properly unshare skbs in phonet_rcv()

Ivaylo Dimitrov reported a regression caused by commit 7866a621
("dev: add per net_device packet type chains").

skb->dev becomes NULL and we crash in __netif_receive_skb_core().

Before above commit, different kind of bugs or corruptions could happen
without major crash.

But the root cause is that phonet_rcv() can queue skb without checking
if skb is shared or not.

Many thanks to Ivaylo Dimitrov for his help, diagnosis and tests.
Reported-by: NIvaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Tested-by: NIvaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Remi Denis-Courmont <courmisch@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/phonet/af_phonet.c

@@ -377,6 +377,10 @@ static int phonet_rcv(struct sk_buff *skb, struct net_device *dev,
 	struct sockaddr_pn sa;
 	u16 len;
 
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb)
+		return NET_RX_DROP;
+
 	/* check we have at least a full Phonet header */
 	if (!pskb_pull(skb, sizeof(struct phonethdr)))
 		goto out;